Fifty one thousand per head's not stiff (4)
I believe the answer is:
limp
'not stiff' is the definition.
(I've seen this in another clue)
'fifty one thousand per head's' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'one' becomes 'i' (Roman numeral).
'thousand' becomes 'm' (Roman numeral for a thousand).
'head's' suggests taking the first letters (I've seen 'head' mean this).
The first letter of 'per' is 'p'.
'l'+'i'+'m'+'p'='LIMP'
(Other definitions for limp that I've seen before include "Walk with an uneven stop" , "Walk with difficulty" , "Slack, wilted" , "obstacle to smooth progress" , "Lacking stiffness" .)